Improvement in Aluminum Complexes Bearing Schiff Bases in Ring-Opening Polymerization of ε‑Caprolactone: A Five-Membered-Ring System

A series of five- and six-membered-ring Al complexes bearing Schiff bases was synthesized and their application to the ring-opening polymerization of ε-caprolactone (CL) was studied. The five-membered-ring Al complexes have been shown to have a significantly higher polymerization rate than six-membered-ring Al complexes (2–3 fold for CL polymerization). The X-ray data revealed that the Al center of a five-membered-ring Al complex is farther from the Schiff base ligand than is that of a six-membered-ring Al complex. The results of density functional theory calculations also suggest that more space around the Al center of five-membered-ring Al complexes may reduce the steric repulsion in CL polymerization and increase the catalytic activity of five-membered-ring Al complexes.